This invention relates to an improved process for catalytically hydrogenating acyclic and cyclic carboxylic acid anhydrides to esters and lactones, respectively, wherein the catalyst is a ruthenium organophosphorous complex.
U.S. Pat. No. 3,957,827 to J. E. Lyons, issued May 18, 1976, describes the selective catalytic hydrogenation of acyclic and cyclic carboxylic acid anhydrides to esters and lactones wherein the catalyst is a ruthenium organophosphorus complex of the formula (I): ##STR1## where X is hydrogen, chlorine, bromine, iodine or lower alkyl; n is an integer of from 0 to 2, but when n is 2, X may be the same or different; L is a neutral ligand, olefin or CO; y is an integer of from 0 to 3, but when y is 2 or 3, L may be the same or different; R.sub.6, R.sub.7 and R.sub.8 are lower alkyl, cycloalkyl of from 5 to 15 carbon atoms, aryl, benzyl or a bidentate ligand, and each of the R groups may be the same or different; and x is an integer of from 1 to 3. The reaction occurs readily under mild reaction conditions in homogeneous solution and is characterized by good selectivity and yield, and does not proceed beyond ester or lactone formation.
When X in formula I is a hydrogen halide such as HCl or dihalide such as Cl.sub.2, or when the catalyst of formula I is formed in situ in the reaction mixture such that a hydrogen halide or a dihalide remains in the reaction mixture, the hydrogen halide contained in the reaction mixture or liberated during the hydrogenation reaction can catalyze hydrolysis of starting carboxylic acid anhydride to the corresponding acid. This hydrolysis reaction thus competes with production of the ester or lactone.
Additional competition results from formation of water as a byproduct of the hydrogenation, since the water also promotes hydrolysis of starting anhydride to the acid form. The effect of the latter hydrolysis reaction can be minimized or somewhat offset by the addition of water scavengers such as a molecular sieve or MgSO.sub.4, in combination with recovery, dehydration and recycling of any acid which neverless may form. However, the catalytic hydrolysis effected by the hydrogen halide will persist.
A similar problem occurs in the hydrogenation reaction with an improved version of the ruthenium organophosphorus catalyst, which hydrogenation reaction and catalyst are described in the copending application of Chao-Yang Hsu and James E. Lyons entitled "Hydrogenation of Carboxylic Acid Anhydrides to Lactones or Esters Using a Ruthenium Trichlorostannate Catalyst," which application is filed simultaneously with the present application and is incorporated herein by reference. However, in the improved process of the copending application the catalyst is more active than the catalyst of U.S. Pat. No. 3,957,827, thereby favoring the hydrogenation reaction over the competing hydrolysis. The catalyst of the copending application has the following formula (II): ##STR2## wherein X is hydrogen, chlorine, bromine, iodine or lower alkyl; m is the integer 1 or 2; n is an integer of from 0 to 3 but when n is 2 or 3, X may be the same or different; M is P, As or Sb; R.sup.6, R.sup.7 and R.sup.8 independently are lower alkyl, cycloalkyl, aryl, benzyl or a bidentate ligand; x is an integer of from 1 to 4 but when x is 2 or more, M may be the same or different; L is a neutral ligand, olefin, CO or (R.sup.9).sub.2 CO wherein R.sup.9 is lower alkyl; y is an integer of from 0 to 3 but when y is 2 or 3, L may be the same or different; and the sum of x and y is at least 2.
As in the case of the ruthenium catalyst of U.S. Pat. No. 3,957,827 the source of the hydrogen halide hydrolysis catalyst can be either those forms of the ruthenium catalyst which can liberate the hydrogen halide during the hydrogenation reaction or can be free hydrogen halide in the catalyst when the catalyst is formed in situ in the reaction mixture.